Antarctica is almost entirely covered in permanent ice and snow, and permanent or seasonal ice-free areas of exposed rock are rare (21,700 km2 out of 14 million km2) (Bockheim 2015, Burton-Johnson et al. 2016). Ice-free areas are either isolated mountain tops, mountain ranges, dry valleys, exposed coastal fringes or offshore subantarctic islands.
Air temperatures vary with latitude and altitude. At coastal locations on subantarctic islands, average air temperatures are generally around 2–6 °C, with a range of less than 10 °C (Bergstrom et al. 2006a). Coastal average air temperatures around Australia’s Antarctic stations are milder than the interior and can rise to more than 0 °C in summer, but drop to less than –30 °C in winter. The region between 60°S and 70°S is the cloudiest on our planet, with 85–90 per cent cloud cover throughout the year (Bargagli 2005). Winds that are generated in the interior of the continent drive cold, dense air towards the coast. Smooth ice surfaces on the ice plateau and steep slopes at the coast reduce friction and intensify katabatic winds, which are strongest at the edge of the continent (often 180 kilometres per hour or more).
The main requisite for life in Antarctica is the availability of liquid water (Bergstrom et al. 2006b), which is mediated by solar radiation, temperature, and ice or snow cover (Convey et al. 2014). Most life occurs in ice-free areas. More than 99 per cent of Antarctica’s biodiversity is concentrated in areas that are permanently ice-free. In both the onshore and island realms, terrestrial habitat can be considered as islands or archipelagos surrounded by ice and/or sea, with most terrestrial biodiversity located near the coast (Frenot et al. 2005).
Within ice-free areas in Antarctica, terrestrial habitats include soft sediments (clays, sands, gravels), and habitats within, under and on top of exposed rock. In some areas, vegetation occurs where ancient penguin colonies used to be, or on extensive humic material derived from plants and built up over thousands of years. Visible life is mainly, although not completely, confined to lower altitude areas in coastal regions (Convey et al. 2014). In the subantarctic, many terrestrial ecosystems are built on extensive and often ancient peats, as well as on soil, gravel and rocks. Lakes and drainage systems are an important element of most ice-free areas of Antarctica and the subantarctic islands. A network of subglacial lakes also occurs across the Antarctic continent, and the lakes are likely to have biota in them (Kennicutt & Siegert 2011).
Most ice-free areas in Antarctica are young (less than 10,000 years old), but some ice-free refuges have been present for millions of years, allowing life to persist for multiple glacial cycles. Recent research has highlighted the role of volcanic areas in sustaining continuous conditions for life during past ice ages (Fraser et al. 2014). By using continent-wide biodiversity databases and ecological informatic approaches, recent biodiversity analyses have shown substantial spatial diversity across the continent, with 15 distinct ecoregions now recognised on the continent itself, and another 8 across the Southern Ocean islands (Terauds et al. 2012). Diversity within species, at local spatial scales of hundreds of metres to hundreds of kilometres, is also being discovered through phylogeographic approaches, reflecting the effects of both older glacial history and more recent events.
Higher vertebrates that use ice-free areas in Antarctica for nesting include Adélie penguins, and flying seabirds such as Antarctic petrels (Thalassoica antarctica) and snow petrels. Some seals use coastal Antarctic beaches as haul-out areas and fast ice (sea ice adjacent to land) for breeding. The subantarctic islands are major breeding and resting grounds for many species of penguin, flying seabirds (such as albatrosses and petrels), and fur and elephant seals. Most vertebrates, such as seabirds, penguins and seals, rely on the ocean for food.
Although the species richness of higher plants and insects is low in Antarctica, plants such as mosses and lichens are relatively well represented (Peat et al. 2007), as are invertebrates such as springtails, nematodes, tardigrades and mites (Velasco-Castrillon et al. 2014). Recent studies have also highlighted the diversity of microbial life in terrestrial Antarctica, with high-throughput DNA sequencing and metagenomic techniques (both used to analyse genomes) clearly showing that microbial diversity is much higher than previously thought (Fierer et al. 2012). The microbiotic communities (cyanobacteria, bacteria, fungi, viruses) are species-rich compared with communities elsewhere, and exist in streams, lakes, moss cushions and soil. Many microorganisms, such as some species of diatoms and cyanobacteria, are endemic to Antarctica (Vyverman et al. 2010). The environmental conditions that these species face across much of the continent are often described as some of the harshest on the planet. To survive these conditions, many of these species have developed unique physiological adaptations, including the ability to survive desiccation or freezing.
The subantarctic islands represent some of the rarest ecosystems on Earth. Subantarctic islands have a range of origins and ages, from remnant Gondwanan continental elements (South Georgia) to sea-floor material (Macquarie Island) that uplifted 600,000 years ago. There are at least 16 active volcanoes in the subantarctic and Antarctica, including (Bergstrom et al. 2006b):
- Big Ben on Heard Island in the southern Indian Ocean
- Mount Erebus and Mount Melbourne in the Transantarctic Mountains adjacent to the Ross Sea in Antarctica.
Terrestrial ecosystems are isolated from each other, and their floral and faunal communities are less complex than those at lower latitudes (Bergstrom & Chown 1999) or the Arctic region. For example, there are 900 species of vascular plants in the Arctic (Bliss 1971), compared with 2 species in the Antarctic (Komárková 1985).
Flora and invertebrate fauna are well developed on the subantarctic islands (see Box ANT6), with vegetation types ranging from tundra-like, sparse fellfields on the uplands to lush grasslands and herbfields on the coast. On the New Zealand subantarctic islands, lower-lying areas also have shrub, heath and coastal woodlands. Species diversity increases with decreasing latitude, but it is still lower in the subantarctic zone than in temperate regions, although species are often highly abundant. Compared with the terrestrial flora of Antarctica, subantarctic vascular plants are diverse and include tens of flowering plant species, including megaherbs and grasses. Mosses and liverworts are also a significant component of the landscape (Bergstrom & Chown 1999). Microbes, algae, fungi and lichens are also critical elements of subantarctic ecosystems. Trees and shrubs are absent from the Australian subantarctic islands, but do occur on other subantarctic islands. The faunal diversity is dominated by invertebrates and includes microarthropods, such as springtails and mites, and insects, including beetles and flies.
In Australia, Macquarie Island was the only breeding site for albatrosses or giant petrels where introduced species—rabbits, rats and mice—were present. An eradication program was successfully completed in 2012, and no rabbits or rodents have been sighted since. The populations of nontarget species most affected by the baiting under the eradication program appeared to recover well, except for skuas. They relied on rabbits as a major source of prey, which probably kept the skua population at a level well above its natural state. The vegetation is recovering quickly, and many seabird species are returning to the island to breed (see Box ANT1).
Long-term programs continue to monitor vegetation changes, and the abundance and distribution of nesting seabirds. To maintain the current status of Macquarie Island as free from introduced mammals, it is vital to implement rigorous biosecurity procedures. In 2013, new measures were introduced to facilities (such as the cargo facility at Hobart wharf) and to procedures. Both state and Australian government agencies are working together to achieve the highest level of biosecurity for transport of goods and people to Macquarie Island.